Silicon carbide semiconductor materials are currently used in high power electronic applications because of their wide bandgap, high breakdown voltage, high electron mobility, and high thermal conductivity. Silicon carbide semiconductors have advantages over silicon and other semiconductor material in high power applications. However, the functional advantages of silicon carbide semiconductors is offset by the cost of manufacture, which in many applications is greater than silicon and other devices. Greater use of silicon carbide semiconductors will depend on whether the cost of manufacture can be decreased.
Generally, silicon carbide semiconductor devices are fabricated by the steps of 1) growing a silicon carbide crystal, also referred to as a boule; 2) slicing the boule into silicon carbide wafers; 3) polishing the wafers to thereby produce a silicon carbide substrate; 4) growing an epitaxial layer on the silicon carbide substrate; and 5) fabricating a device utilizing the epitaxial layer alone or in combination with the substrate. The present invention is generally directed towards the first step of growing a silicon carbide crystal of the 4H polytype.
Silicon carbide crystals occur in an abundance of polytypes (e.g. 2H, 4H and 6H) which are individual crystal structures differing mainly in the repetition pattern of the atomic stacking sequence, see FIG. 1. Each polytype exhibits different electronic properties (i.e. bandgap, effective mass, carrier mobility etc.). Experiments have shown that in high frequency applications, the 4H polytype has advantages over 2H and 6H polytype. Compared to 6H polytype, the 4H polytype exhibits a factor of seven higher electron mobility and significantly less anisotropy in its electrical properties.
SiC Boules can be grown using a physical vapor transport system. As shown in FIG. 2, a physical vapor transport system may include a growth chamber 10 filled with an ambient gas 17 and having a seed holder 12, a seed crystal 14 fixed to the seed holder 12 and a source material 18. Operationally, the growth chamber 10 is heated to cause sublimation of the source material 18 whereby the source material essentially evaporates into the ambient gas 17 and thereafter contacts the seed crystal 14 causing crystal growth from the surface of the seed crystal 14. The crystal grows in a direction from the seed crystal 14 towards the source material 18. When silicon carbide boules are grown in the &lt;0001&gt; direction (the orientation normally employed for electronic devices), the seed crystal 14 is silicon carbide having a silicon face and a carbon face, and the source material 18 is silicon carbide. The composition of the ambient gas 17 determines the doping characteristics of the resulting boule. For example, a highly conductive silicon carbide substrate would be fabricated from a boule doped with nitrogen. Such a boule would be grown in an ambient gas 17 containing nitrogen.
It is undesirable to produce boules having mixtures of different polytypes. While it is necessary to grow the silicon carbide crystal from the carbon face of the seed crystal, such is not sufficient to achieve long boules. In practice in prior art systems where 4-H polytype boules are desired, the yield of the 4-H polytype typically decreases as the length of the boule increases. That is, as the length of the boule increases the likelihood of producing a 4-H/6-H polytype mixture increases. Therefore, silicon carbide crystals having a high percentage of 4-H polytype tend to be relatively short in length. Therefore, it is desirable to provide a method of growing boules where the yield of the 4-H polytype in the resulting boule is not affected by the length of the boule. In addition, in maximizing the 4-H polytype yield, it is also desirable to maximize the growth rate of the boule.